Re: [Qemu-devel] [PATCH v2] i386: Add new Hygon 'Dhyana' CPU model

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Apr 16, 2019 at 09:16:05AM +0100, Daniel P. Berrangé wrote:
> On Mon, Apr 15, 2019 at 05:39:17PM -0300, Eduardo Habkost wrote:
> > On Sat, Apr 13, 2019 at 10:54:40AM +0800, Pu Wen wrote:
> > > Add a new base CPU model called 'Dhyana' to model processors from Hygon
> > > Dhyana(family 18h), which derived from AMD EPYC(family 17h).
> > > 
> > > The following features bits have been removed compare to AMD EPYC:
> > > aes, pclmulqdq, sha_ni
> > > 
> > > The Hygon Dhyana support to KVM in Linux is already accepted upstream[1].
> > > So add Hygon Dhyana support to Qemu is necessary to create Hygon's own
> > > CPU model.
> > > 
> > > Reference:
> > > [1] https://git.kernel.org/tip/fec98069fb72fb656304a3e52265e0c2fc9adf87
> > > 
> > > Signed-off-by: Pu Wen <puwen@hygon.cn>
> > 
> > Thanks for the patch.
> > 
> > I'm wondering if we should let the CPU model be used only on
> > Hygon hosts, to avoid confusion.
> 
> Why should we artificially restrict it ?  All the other CPUs are able to
> be used on any host that is able to support the feature list required by
> the CPU model. If some other host has sufficient features to run Dhyana
> the CPU model we shouldn't block it.

Running it on Intel or AMD hosts will create a frankenstein CPU
with vendor=AuthenticAMD|GenuineIntel but with
family/model/stepping/model_id values that make sense only on
Hygon CPUs.  I don't see why this is preferable to simply telling
the user that the CPU model is unavailable.

If somebody really needs that specific set of features and know
they are runnable on their AMD host, they can easily run
"-cpu EPYC,+aes,+pclmulqdq,+sha-ni".

We have the same issue with Intel & AMD CPUs.  The only reason we
don't prevent this with AMD or Intel CPU models is our huge fear
of breaking backwards compatibility.

-- 
Eduardo